Faulty actuator on diverter valve?

22 May 2004
Reaction score
United Kingdom
I have a myson 3 port diverter valve which almost works ok - it turns to central heating or hot water and central heating by motor. When central heating is turned off though the spring does not return the valve to the hot water only position UNTIL the fuse is switched off. If the central heating is just switched off by the programmer or by the thermostat an electrical buzzing can still be heard in the actuator and it gets hot. As I know the programmer and thermostat are working correctly, I assume it is the actuator itself where the fault lies. Any opinions would be welcome before I buy a new actuator. Thanks :confused:
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From THIS thread:

"It is normal for the diverter valve not return to its resting position when the cylinder stat (or programmer) stops calling for DHW before the demand for CH ends. The diverter valve is held in place by the 240V that is applied to its Grey wire, from the programmer's 'DHW Off' connection or the cylinder stat's third (heat satisfied) terminal".
They only seem to buzz when they get a bit old - they get loose enough to vibrate at 50/100 Hz I guess.
Thanks for your reply CH4. Have not been able to reply before now or even been able to look at the actuator etc. I have now looked at the link you suggested and it all made sense, so, as it really seemed to be the actuator that was playing up, this week I bought a new one. However it is exactly the same as the other one, buzzes and gets very hot when it has been in central heating mode. This isn't be normal is it?
It was suggested in the link that the pump could be the problem by drawing excessive current to be drawn. Could you explain a bit more please. The pump was a new one last year, put in by a plumber. How can I check if it's faulty? Or do you have any other suggestions before calling in the cavalry?
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someone answer this........the suspense is killing me.......

i can only think that the actual valve is stiff to turn and straining the motor.

buzzing, whilst head was off did you see if valve moved freely.

mind you its moved over far enough to run pump & boiler.......

talking to me self now.... :eek:
Yes I thought the valve might have been stiff so I operated the actuator not connected to the valve and saw that it turned correctly to each position but still made the buzzing once switched off at the thermostat and/or programmer. Valve also turned easily when moved with pliers - no strain at all! Any other ideas desperately appreciated. :confused: :confused:
i still dont know then :D
did you slave in another programmer? you said it was ok.how are you sure?.. and is the tank stat ok?
i say that cos thats all about thats left in the sytem..i think. .......

have you tried disconncting the pump brown wire and see if noise stops?

other than that im stumped :rolleyes:
Thanks for reply Wilhelm...As the actuator only makes a noise and gets hot in central heating mode, I assumed it wasn't the tank stat. I had tried moving temperature guage on tank a while ago and it seemed to stop boiler and pump correctly as I reduced the temperature. I'll try taking out the wire to the pump tomorrow and see if that stops noise. Wish I knew if it had made the noise before the new pump was installed last year. I think it's not the programmer as I had swapped it over with a neighbours when I first noticed problem (after sitting in their airing cupboard listening for buzzing!!) and actuator still made the same noise and heated up when put in my system. Totally stumped this end too :(
Can I suggest using finger and thumb to check resistance of spindle instead of pliers, then re-check after applying WD 40
or tried turning pump speed down?

err clutching @ straws now.......... :eek:

or tie a pillow around it.....

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